The requirement for sensing position, speed or acceleration is growing, particularly in the automotive field. Anti-lock braking systems, traction control systems, electric power steering, four-wheel steering and throttle control are all examples of functions that can use such sensing means.
For such applications, it is desirable to have a position sensor, wherein speed and acceleration may be derived from a position signal. Such a sensor should be rugged and reliable, small and inexpensive, capable of low (including zero) speed sensing and relatively immune to noise, such as electromagnetic field interference, from the other systems used in an automobile.
A commonly known type of position sensor is a semiconductor magnetoresistive sensor. Such a sensor comprises a magnetic circuit that includes two basic parts. One of these parts, typically kept stationary, includes a semiconductive sensing element that is sensitive to the level of magnetic flux density passing through its surface. This stationary element further includes a permanent magnet for creating a bias flux. The other of the two parts, commonly referred to as the exciter, includes a high magnetic permeability element with a series of teeth that moves with relation to the stationary element. As the teeth move past the stationary element, the reluctance of the magnetic circuit and the magnetic flux density through the sensing element vary continually, corresponding to the relative position of the teeth.
An illustrative example of such a semiconductor magnetoresistive sensor is taught in U.S. Pat. No. 4,926,122 to Schroeder et al. entitled, "High Sensitivity Magnetic Circuit," issued May 15, 1990 and assigned to the same assignee of the present patent application. The Schroeder sensor features a simple, planar geometry that makes it amenable for batch processing at a relatively low cost, while still providing sensitivities which are appreciably higher than the prior art structures. The Schroeder sensor more than satisfactorily meets the objectives of such a sensor.
However, as the use of these semiconductor magnetic position and speed sensors is increased, it would be desirable to further reduce the cost of the sensor, so as to make it even more attractive for automotive use. As with many other types of semiconductor devices, the cost of packaging the magnetic semiconductor chip into a magnetic position sensor generally exceeds the cost of the semiconductor chip itself, often by a factor of many times. Accordingly, it would appear that a substantial cost saving could be realized by the use of a lower cost package.
For best sensor performance in this type of magnetoresistive sensor which contains a small permanent magnet in addition to the semiconductor chip, the semiconductor chip should be attached directly to the surface of the permanent magnet. Currently, in order to accomplish this, the packaging process involves both macroelectronic assembly techniques for construction of the sensor body housing the permanent magnet, and microelectronic assembly techniques for attachment of the semiconductor chip. Generally, these two assembly techniques are relatively incompatible because of the extreme difference in dimensions for each of the operations and, therefore, cannot be performed concurrently. Thus, the sensor manufacturers typically employ a two-step packaging process. First, the semiconductor chip is packaged as a semiconductor device using microelectronic packaging technology. Then, the packaged device is repackaged by the sensor manufacturer into the resultant sensor for the particular application.
A significant shortcoming associated with this current packaging approach is that it suffers from too many steps and components. In addition, the thickness of the packaged semiconductor sensing element is significantly larger than that of the active components within the sensor. This is disadvantageous in that the thickness of the sensing element requires that the permanent magnet be positioned farther from the toothed exciter wheel, and the farther apart the permanent magnet and the exciter wheel, the weaker the magnetic field in the air-gap therebetween. Thus, the sensitivity of the sensor is diminished. Also, because of the unnecessarily large size of the packaged semiconductor element, intricate sensor geometries which would make high resolution and/or miniature applications possible cannot be considered.
Therefore, it would be desirable to utilize semiconductor chip packaging technology, which is highly accurate and automated, for the production of these and other types of sensors. The use of semiconductor packaging techniques would result in a package of reduced size, and probably also reduced cost, as compared to conventional techniques for manufacturing these types of sensors. In order to utilize semiconductor packaging techniques, the permanent magnet and the sensor terminals should be provided in a form which is compatible with the placement and bonding equipment used to attach the semiconductor sensing element to the sensor body.